Question

I want to create a controller that depends on the class of a given instance of a model

 -(BaseController *)getControllerForModel:(Model *)model
    {
     BaseController *controller = nil;
     Class controllerClass = [BaseController class]; //the default value

        //find the right controller
     if ([model isMemberOfClass:[ModelClass1 class]]) 
      controllerClass = [ConcreteController1 class];
     else if ([model isMemberOfClass:[ModelClass2 class]])
      controllerClass = [ConcreteController2 class];
     else if ([model isMemberOfClass:[ModelClass3 class]])
      controllerClass = [ConcreteController3 class];
...
     else if ([model isMemberOfClass:[ModelClassX class]])
      controllerClass = [ConcreteControllerX class];
     else
      Trace(TRACELEVEL_WARNING, @"Unrecognized model type: %@", NSStringFromClass([model class]));

     //Now instantiate it with the model
     controller = [[[controllerClass alloc] initWithModel:model] autorelease];
     return slotController;
    }

I want to find a more flexible solution to this and thought of having a dictionary, which maps Model-Classes to Controller-Classes and then NSClassFromString could give me the right instance.

My question is this: Is NSClassFromString using much of my applications performance if i use it several times (say, 100 times at once)? Or would it be about as fast as the above approach?

Answer 1

A Class is an id, and as such can be added to an NSDictionary. You should try the following:

mModelToControllerMap = [[NSDictionary alloc] initWithObjectsAndKeys:
   [ConcreteController1 class] , [ModelClass1 class] ,
   [ConcreteController2 class] , [ModelClass2 class] ,
   ...
   nil];

Then later:

controllerClass = [mModelToControllerMap objectForKey:[modelInstance class]];
if ( controllerClass ) ...
else ...

If you make it a mutable dictionary, you can then have controllers register for the models they want instead of forcing the base class to be aware of all derived classes.

And a direct answer. At worst, NSClassFromString may need to iterate through all classes to find a match. It could also be a dictionary lookup, but the implementation is not documented and should not be relied on. While doing it a few hundred times probably isn’t too bad, there are better ways.